1. Field of Invention
This invention relates generally to optical telecommunication systems and optical transport networks employed in such systems deploying photonic integrated circuits (PICs) for wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) optical networks and more particularly to a method of calibrating of optical transmitter photonic integrated circuits (TxPICs).
2. Description of the Related Art
If used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
1R—Re-amplification of the information signal.
2R—Optical signal regeneration that includes signal reshaping as well as signal regeneration or re-amplification.
3R—Optical signal regeneration that includes signal retiming as well as signal reshaping as well as re-amplification.
4R—Any electronic reconditioning to correct for transmission impairments other than 3R processing, such as, but not limited to, FEC encoding, decoding and re-encoding.
A/D—Add/Drop.
APD—Avalanche Photodiode.
AWG—Arrayed Waveguide Grating.
BER—Bit Error Rate.
CD—Chromatic Dispersion.
CDWM—Cascaded Dielectric wavelength Multiplexer (Demultiplexer).
CoC—Chip on Carrier.
DBR—Distributed Bragg Reflector laser.
EDFAs—Erbium Doped Fiber Amplifiers.
DAWN—Digitally Amplified Wavelength Network.
DCF—Dispersion Compensating Fiber.
DEMUX—Demultiplexer.
DFB—Distributed Feedback laser.
DLM—Digital Line Modulator.
DON—Digital Optical Network as defined and used in this application.
EA—Electro-Absorption.
EAM—Electro-Absorption Modulator.
EDFA—Erbium Doped Fiber Amplifier.
EML—Electro-absorption Modulator/Laser.
EO—Electrical to Optical signal conversion (from the electrical domain into the optical domain).
FEC—Forward Error Correction.
GVD—Group Velocity Dispersion comprising CD and/or PMD.
ITU—International Telecommunication Union.
MMI—Multimode Interference combiner.
MPD—Monitoring Photodiode.
MZM—Mach-Zehnder Modulator.
MUX—Multiplexer.
NE—Network Element.
NF—Noise Figure: The ratio of input OSNR to output OSNR.
OADM—Optical Add Drop Multiplexer.
OE—Optical to Electrical signal conversion (from the optical domain into the electrical domain).
OEO—Optical to Electrical to Optical signal conversion (from the optical domain into the electrical domain with electrical signal regeneration and then converted back into optical domain) and also sometimes referred to as SONET regenerators.
OEO-REGEN—OEO signal REGEN using opto-electronic regeneration.
OO—Optical-Optical for signal re-amplification due to attenuation. EDFAs do this in current WDM systems.
OOO—Optical to Optical to Optical signal conversion (from the optical domain and remaining in the optical domain with optical signal regeneration and then forwarded in optical domain).
OOO-REGEN—OOO signal REGEN using all-optical regeneration.
OSNR—Optical Signal to Noise Ratio.
PIC—Photonic Integrated Circuit.
PIN—p-i-n semiconductor photodiode.
PMD—Polarization Mode Dispersion.
REGEN—digital optical signal regeneration, also referred to as re-mapping, is signal restoration, accomplished electronically or optically or a combination of both, which is required due to both optical signal degradation or distortion primarily occurring during optical signal propagation caused by the nature and quality of the signal itself or due to optical impairments incurred on the transport medium.
Rx—Receiver, here in reference to optical channel receivers.
RxPIC—Receiver Photonic Integrated Circuit.
SDH—Synchronous Digital Hierarchy.
SDM—Space Division Multiplexing.
Signal regeneration (regenerating)—Also, rejuvenation. This may entail 1R, 2R, 3R or 4R and in a broader sense signal A/D multiplexing, switching, routing, grooming, wavelength conversion as discussed, for example, in the book entitled, “Optical Networks” by Rajiv Ramaswami and Kumar N. Sivarajan, Second Edition, Morgan Kaufmann Publishers, 2002.
SMF—Single Mode Fiber.
SML—Semiconductor Modulator/Laser.
SOA—Semiconductor Optical Amplifier.
SONET—Synchronous Optical Network.
SSC—Spot Size Converter, sometimes referred to as a mode adapter.
TDM—Time Division Multiplexing.
TEC—Thermo Electric Cooler.
TRxPIC—Monolithic Transceiver Photonic Integrated Circuit.
Tx—Transmitter, here in reference to optical channel transmitters.
TxPIC—Transmitter Photonic Integrated Circuit.
VOA—Variable Optical Attenuator.
WDM—Wavelength Division Multiplexing. As used herein, WDM includes Dense Wavelength Division Multiplexing (DWDM).
DWDM optical networks are deployed for transporting data in long haul networks, metropolitan area networks, and other optical communication applications. In a DWDM system, a plurality of different light wavelengths, representing signal channels, are transported or propagated along fiber links or along one more optical fibers comprising an optical span. In a conventional DWDM system, an optical transmitter is an electrical-to-optical (EO) conversion apparatus for generating an integral number of optical channels λ1, λ2, λN, where each channel has a different center or peak wavelength. DWDM optical networks commonly have optical transmitter modules that deploy eight or more optical channels, with some DWDM optical networks employing 30, 40, 80 or more signal channels. The optical transmitter module generally comprises a plurality of discrete optical devices, such as a discrete group or array of DFB or DBR laser sources of different wavelengths, a plurality of discrete modulators, such as, Mach-Zehnder modulators (MZMs) or electro-absorption modulators (EAMs), and an optical combiner, such as a star coupler, a multi-mode interference (MMI) combiner, an Echelle grating or an arrayed waveguide grating (AWG). All of these optical components are optically coupled to one another as an array of optical signal paths coupled to the input of an optical combiner using a multitude of single mode fibers (SMFs), each aligned and optically coupled between discrete optical devices. A semiconductor modulator/laser (SML) may be integrated on a single chip, which in the case of an electro-absorption modulator/laser (EML) is, of course, an EA modulator. The modulator, whether an EAM or a MZM, modulates the cw output of the laser source with a digital data signal to provide a channel signal which is different in wavelength from each of the other channel signals of other EMLs in the transmitter module. While each signal channel has a center wavelength (e.g., 1.48 μm, 1.52 μm, 1.55 μm, etc.), each optical channel is typically assigned a minimum channel spacing or bandwidth to avoid crosstalk with other optical channels. Currently, channel spacings are greater than 50 GHz, with 50 GHz and 100 GHz being common channel spacings.
An optical fiber span in an optical transport network may provide coupling between an optical transmitter terminal and an optical receiver terminal. The terminal traditionally is a transceiver capable of generating channel signals as well as receiving channel signals. The optical medium may include one or more optical fiber links forming an optical span with one or more intermediate optical nodes. The optical receiver receives the optical channel signals and converts the channel signals into electrical signals employing an optical-to-electrical (OE) conversion apparatus for data recovery. The bit error rate (BER) at the optical receiver for a particular optical channel will depend upon the received optical power, the optical signal-to-noise ratio (OSNR), non-linear fiber effects of each fiber link, such as chromatic dispersion (CD) and polarization mode dispersion (PMD), and whether a forward error correction (FEC) code technique was employed in the transmission of the data.
The optical power in each channel is naturally attenuated by the optical fiber link or spans over which the channel signals propagate. The signal attenuation, as measured in dB/km, of an optical fiber depends upon the particular fiber, with the total loss increasing with the length of optical fiber span.
As indicated above, each optical fiber link typically introduces group velocity dispersion (GVD) comprising chromatic dispersion (CD) and polarization mode dispersion (PMD). Chromatic dispersion of the signal is created by the different frequency components of the optical signal travel at different velocities in the fiber. Polarization mode dispersion (PMD) of the signal is created due to the delay-time difference between the orthogonally polarized modes of the signal light. Thus, GVD can broaden the width of an optical pulse as it propagates along an optical fiber. Both attenuation and dispersion effects can limit the distance that an optical signal can travel in an optical fiber and still provide detectable data at the optical receiver and be received at a desired BER. The dispersion limit will depend, in part, on the data rate of the optical channel. Generally, the limiting dispersion length, L, is modeled as decreasing inversely with B2, where B is the bit rate.
The landscape of optical transport networks has change significantly over the past ten years. Prior to this time, most long haul telecommunication networks were generally handled via electrical domain transmission, such as provided through wire cables, which is bandwidth limited. Telecommunication service providers have more recently commercially deployed optical transport networks having vastly higher information or data transmission capability compared to traditional electrical transport networks. Capacity demands have increased significantly with the advent of the Internet. The demand for information signal capacity increases dramatically every year.
In a conventional long haul DWDM optical network, erbium doped fiber amplifiers (EDFAs) may be employed at intermediate nodes in the optical span to amplify attenuated optical channel signals. Dispersion compensation devices may also be employed to compensate for the effects of fiber pulse dispersion and reshape the optical pulses approximately to their original signal shape.
As previously indicated, a conventional DWDM optical network requires a large number of discrete optical components in the optical transmitter and receiver as well as at intermediate nodes along the optical link between the transmitter terminal and the receiver terminal. More particularly, each optical transmitter typically includes a semiconductor laser source for each optical channel. Typically a packaged module may include a semiconductor laser and a monitoring photodiode (MPD) to monitor the laser source wavelength and intensity and a heat sink or thermal electric cooler (TEC) to control the temperature and, therefore, wavelength of the laser source. The laser sources as well as the optical coupling means for the output light of the laser source to fiber pigtail, usually involving an optical lens system, are all mounted on a substrate, such as a silicon microbench. The output of the laser pigtail is then coupled to an external electro-optical modulator, such as a Mach-Zehnder lithium niobate modulator. Alternatively, the laser source itself may be directly modulated. Moreover, different modulation approaches may be employed to modulate the external modulator, such as dual tone frequency techniques.
The output of each modulator is coupled via an optical fiber to an optical combiner, such as, an optical multiplexer, for example, a silica-based thin film filter, such as an array waveguide grating (AWG) fabricated employing a plurality of silicon dioxide waveguides formed in a silica substrate. The fibers attached to each device may be fusion spliced together or mechanically coupled. Each of these device/fiber connections introduces a deleterious, backward reflection into the transmitter, which can degrade the channel signals. Each optical component and fiber coupling also typically introduces an optical insertion loss.
Part of the cost of the optical transmitter is associated with the requirement that the optical components also be optically compatible. For example, semiconductor lasers typically produce light output that has a TE optical mode. Conventional optical fibers typically do not preserve optical polarization. Thus, optical fiber pigtails and modulators will transmit and receive both transverse electric (TE) and transverse magnetic (TM) polarization modes. Similarly, the optical combiner is polarization sensitive to both the TE and TM modes. In order to attenuate the effects of polarization dispersion, the modulator and the optical combiner are, therefore, designed to be polarization insensitive, increasing their cost. Alternatively, polarization preserving fibers may be employed for optically coupling each laser source to its corresponding modulator and for coupling each modulator to the optical combiner. Polarization preserving fibers comprise fibers with a transverse refractive index profile designed to preserve the polarization of an optical mode as originally launched into a fiber. For example, the fiber core may be provided with an oblong shape, or may be stressed by applying a force to the fiber to warp the refractive index of the waveguide core along a radial or cross-sectional lateral direction of the fiber, such as a PANDA ™ fiber. However, polarization preserving fibers are expensive and increase packaging costs since they require highly accurate angular alignment of the fiber at each coupling point to an optical component in order to preserve the initial polarization of the channel signal.
A conventional optical receiver also requires a plurality of discrete optical components, such as an optical demultiplexer or combiner, such as an arrayed waveguide grating (AWG), optical fibers, optical amplifiers, and discrete optical detectors as well as electronic circuit components for handling the channel signals in the electrical domain. A conventional optical amplifier, such as an EDFA, has limited spectral width over which sufficient gain can be provided to a plurality of optical signal channels. Consequently, intermediate OEO nodes will be required comprising a demultiplexer to separate the optical channel signals, photodetector array to provide OE conversion of the optical signals into the electrical domain, 3R processing of the electrical channel signals, EO conversion or regeneration of the processed electrical signals, via an electro-optic modulator, into optical signals, optical amplifiers to amplify the channel signals, dispersion compensators to correct for signal distortion and dispersion, and an optical multiplexer to recombine the channel signals for propagation over the next optical link.
There is considerable interest in DWDM systems to increase both the data rate of each signal channel as well as the number of channels, particularly within the gain bandwidth of the EDFA. However, increasing the channel data rate necessitates increasing the number of intermediate nodes along the optical path to provide the required signal dispersion compensation and amplification. Increasing the number of channels requires precise control of channel assignment and more precise control over signal dispersion, which dramatically increases the complexity and cost of the fiber-optic components of the system. A further complication is that many pre-existing optical networks use different types of optical fibers in the different optical links of the optical network having, therefore, different dispersion effects over different fiber lengths. In some cases, the wavelengths of the optical channels generated at the optical transmitter may not be optimal for one or more optical links of the optical span.
What is desired are improved techniques to provide DWDM optical network services through improved, integrated optical network components and systems.